Expert Systems are the most important of the applications of Artificial Intelligence in the commercial and defense sectors. Topics: the rapid transition of the Expert System technology from laboratories to societal use; what is in an Expert System; what is Knowledge Engineering. Case studies of commercial application in: diagnosis and repair; interpretation of data; manufacturing planning and control; financial services; engineering design, etc. The sources of benefit from Expert Systems. The magnitude of these benefits. What an organization needs to do to realize the benefits. A "what" rather than a "how to build systems" orientation aimed for a broad interdisciplinary audience